liquidwater maystick onhydrophobic surfaces · liquidwater maystick onhydrophobic surfaces suman...

Indian Institute of Technology Kharagpur, India

Liquid Water may Stick on HydrophobicSurfaces

Suman Chakraborty

ProfessorDepartment of Mechanical Engineering, IIT Kharagpur, India

July, 2016


Common PerceptionA surface can be classified as

WettingNon-wetting

Depending on the spreading characteristics of a droplet of water thatsplashes on the surface.

The behavior of fluid on a solidsurface under static and dynamicconditions are usually clubbed

together

• On a wetting surface (hydrophilic), liquid water is believed to adhere tothe surface causing multilayer sticking.• On a non-wetting surface (hydrophobic), water is believed to glideacross the surface leading to slip


Wettability and Molecular interactions

The molecular interactions apart from Coulombic (not electrostatic)and gravitational (not relevant at small scales) , all the other

interactions are classified as van der Waals interactions

• van der Waals interactions exist between any two bodies irrespective oftheir size or polarity.

• They are long range interactions where the interaction energy w(r) variesas 1/r6 with separation distance of r between the interacting bodies.

• Three types of van der Waals forces occur, namely,

1. Orientation force (Keesom interaction)

2. Induction force (Debye interaction)

3. Dispersion force (London interaction)

The upscaled palpable output of theses molecular interactions is

Wettability


WettabilityApplications

• Waxed surfaces(Prevention of rust/easy drying surfaces)• Wind shields(Self Cleaning surfaces)

• Plant leaves(Irrigation/water cycle)• Sand Castles on a beach(Construction)


Wettability:Forces at the Contact Line

: solid-vapor surface tension

: solid-liquid surface tension

: vapor-liquid surface tension

equilibrium contact angle :Young Dupré relation

non wetting liquid : > 90°

partially wetting liquid : < 90°

perfectly wetting liquid : =0°

LV

LV

SV

SL

SL

SV

cosSV SL LV


• Some of the common naturalHydrophobic materials arewaxes, oil and fats.

Hydrophobicity comes also from the greek word Hydro(water) and Phobicity (fear) it refers to the physical property ofa material that repels a mass of water.

WHAT IS MEANT BY HYDROPHOBICITY ?

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The evaluation of hydrophobicityis made through water contactangle measurements.

On a hydrophobic surface, waterdroplet would form a largecontact angle.


Hydrophobicity, Slip, and Slip Length

slip length, quantification of slip to be usedas a boundary condition

wshear stress at solid surface

slNavier(1823)

Problems Addressed by assuming slip:1. Apparent moving/slipping contact line2. Infinite Energy Dissipation/Contact line

Singularity(Huh & Scriven paradox)

With low solid-fluid interaction energy and highsurface area to volume ratios, there is inefficientmomentum transfer between the wall and fluid,resulting in non-zero velocities any ‘no-slip’continuum boundary

wwall

uy

/s wwall

ul u

y


Slip Boundary Condition for Liquids??• Because of sufficient intermolecular forces of attraction between the

molecules of the solid surface and a dense medium such as the liquid, it isexpected that the liquid molecules would remain stationary relative to the solidboundary at their points of contact.

• Only at very high shear rates (typically realizable only in extremely narrowconfinements of size roughly a few molecular diameters), the straining may besufficient enough in moving the fluid molecules adhering to the solid byovercoming the van der Waals forces of attraction.

• Another theory argues that the no-slip boundary condition arises due tomicroscopic boundary roughness, since fluid elements may get locally trappedwithin the surface asperities. If the fluid is a liquid then it may not be possiblefor the molecules to escape from that trapping, because of an otherwisecompact molecular packing.


What Happens for Carbon Nanotubes?

• Researchers have demonstrated that the rate of liquid flow through amembrane composed of an array of aligned carbon nanotubes mightturn out to be four to five orders of magnitude faster than thatpredicted from classical fluid-flow analysis.

• They attributed this phenomenon to an apparently frictionlessinterfacial condition at the carbon-nanotube wall.

• Water is likely to be able to flow fast through hydrophobic single-walled carbon nanotubes; the primary reason being the fact that theprocess creates ordered hydrogen bonds between the watermolecules.

• Ordered hydrogen bonds between water molecules and the weakattraction between the water and smooth carbon nanotube graphitesheets, as well as the rapid diffusion of hydrocarbons are qualitativelyattributed to the fundamental scientific origin of reduced frictionalresistances encountered in such systems.


Why would we want to obtain Stick on a

Hydrophobic surface?

• Switching substrate wettability between hydrophobicity and hydrophilicity canfind a huge number of applications.

The intrinsicsurface

Hydrophobicinteractions gives

The Stick obtainedgives

• Minimum contact withwater• Rapid interfacialmovement of water

• Localized surfaceenergy driven transport• Targeted flushing


Self Cleaning Surfaces

Self cleaning window panes and bathrooms tiles whichrepel water that ‘stains’ but is wetted by water that ‘cleans’.

• Imagine having a switch which can makethe floors and walls of the bathroomhydrophilic while cleaning and hydrophobicwhile in use.

• Hydrophobic coatings for electronicequipments which have low porosity as wellas high sensitivity

• Did you ever wonder why is it difficult tomake spectacle lenses scratch proof andmoisture repellent at the same time?


Mixing

• At micro and nanoscales, the low Reynolds numberprevents introduction of turbulence.• Active mixing at these scales would require hugeamount of external energy and is not feasible.• Existing passive mixing techniques often involve‘meddling’ with the fluid and the flow and is often notdesirable.• Alternate slip and stick can generate passive mixingover a very confined conduit without altering thebackground flow properties.


Prevention of Fouling

• Filtration membranes are usually hydrophobic, leading torestricted water access to the interface.• Ion-selective membranes often attract ionic impuritieswhich adhere to the surface.• The stagnation due to low water flushing at the surfaceleads to fouling of the membranes.• Switchable wettability can lead to temporary surge inwater flow near the membranes leading to flushing of themembranes.


Pumpless Transport

• Most polymeric materials available for lab use arehydrophobic.• Such material as required is usually treated with oxygenplasma which gives temporary hydrophilization.• Permanent hydrophilization techniques are oftenexpensive and difficult to implement at small scales• Hydrophilization of naturally hydrophobic materials using‘non-invasive’ methods can promote passive (pumpless)transport in certain parts of the fluid circuit


Delinking Slip from Intrinsic Wettability

151

Is it possible to tune the wettability,back and forth, on the same substrate

depending on the requirement?

• Wettability and contact angle, although often used assynonyms at macroscopic levels, have different originsand manifestations at molecular levels.• Often contact angle hysteresis rather than contactangle is used to describe the “substrate wetting” underdynamical conditions.• Under dynamic conditions, the wettability is not just afunction of the chemical properties of the solid substratebut also the physical structure of the interface

Ref: C. Bakli, S. Chakraborty, Nano Letters, vol. 15, pp. 7497-7502, 2015


Effect of Lattice Structure on Slip

16

• The interparticle distance or the lattice parameter is the physicaldimension of the unit cell in a crystal lattice and is denoted by D.• Smaller value of D means the atoms/particles are closely packed inthe lattice and vice versa• A closely packed lattice would have adsorption and desorption sitesalso closer to each other.• Water under static conditions forms H-bonds with other watermolecules and the surface, attaining the most energetically favorableconfiguration.• Under the action of a external shear, these water moleculesrearrange themselves to the next energetically favorableconfiguration.


Overview of Molecular Dynamics Simulations:General Algorithm

InitialConfiguration

Potential

CalculateForce

UpdatePositions

r(t)

Obtainv(t),a(t)

Trajectory


Initialization• Initial coordinates of the atoms/molecules/particles can be

obtained as:Experiment/Theoretical models- Geometry set using the existing

data available for the structure.

Lattice arrangement- Particles placed in a regular lattice structuredepending on the bulk material density.

Monte Carlo Technique- An initial Monte Carlo simulation is done toobtain a local energy minima for the structure.

• Initial velocities may be assigned as:Maxwell-Boltzmann distribution with the velocity distributioncorresponding to the initial temperature of the system.

Zero-Velocity initialization.


Equilibration

• Analysis can be done with equilibrated system only.

• One needs to check various criteria to judge thepoint of reaching equilibrium.

Total energy fluctuation (only for NVEsimulations).

Boltzmann H Function- Obtained from averageposition and velocity distribution

Pressure and Temperature- The averagefluctuation of these thermodynamic parametersabout stable averages.


Lennard-Jones Model and Contact Angle of Water

• Intermolecular interaction ofwater molecules is expressed asa combination of van der Waalsforces and electrostatic force.

• Considering water as the fluidbrings in the effect of hydrogenbonding and the way it isinfluenced by wettability andshear The Simple Point Charge (Extended)

model (SPC/E) includes the effect ofpermanent and induced dipolesconsidering the polarization effects.U=ELJ+Eelec+Epol where ;

is induced dipole moment andis isotropic polarizability scalar.

212

ipol

i i

E

i

i

20

1 HOH=109.47( ) 0.8476 q(H)=0.4238

o oOHr A

q O


Obtaining Slip from Molecular Velocity Trajectory


Effect of D on slip over Hydrophobic Surfaces

22

• Smaller values of D promote slip on surfaces of any wettability, while increasingD makes slip more unlikely to occur.• Over a hydrophobic surface which is already prone to slip, decrease in D furtherpromotes slip.•The slip enhancement is caused by the coupled effects of:

Increase ease in gliding of water molecules across the adsorption siteswhich are now nearer

Increase in interfacial water density at the adsorption sites due to betterscreening of the hydrophobic effect (thus more number of molecules slipping)

• Slip lengths are observedto increase with decrease inD.• Reduction in D leads tocloser surface adsorptionsites and hence fluidmolecules can easily glidebetween these sites

Indian Institute of Technology Kharagpur, IndiaIndian Institute of Technology Kharagpur

Effect of D on slip over Hydrophobic SurfacesAnalogy

The game of Land and Water (Kumir Danga)

Easier for the kidsto swap islands

with greaternumber of ornearer islands


Effect of Salt Concentration on Slip

24

• The interparticle distance on the solid substrate is observed to changethe interfacial structuration of water molecules. Similarly, solutemolecules can also alter the interfacial structuration and dynamics of fluidmolecules.• Solute molecules tend to compensate the H-bonds of water molecule atthe interface and form local pinning sites.• Any ion as a solute particle would form primary and secondaryhydration shells around itself. Applied shear would now be dislodging theion along with the water molecules in the hydration shells• Dislodging a water molecule from one hydration shell to another wouldrequire breaking of multiple bonds (electrostatic) and require hugeamount of energy, making locomotion of water molecules around an ionconstrained.

For hydrophilic surfaces, water molecules tend to be in the vicinity ofthe interface and ions toward the bulk and thus such quasi-immobile sitesare also in the bulk

For hydrophobic surfaces, ions form these quasi-immobile sites next tothe interface in the water depleted regime.


Effect of Salt Concentration on Slip over HydrophobicSurfaces

25

• Slip lengths are observedto decrease with increase insolute concentration.• Increase in saltconcentration hasanalogous effect to increasein interparticle distance.

The incoming solute molecules form hydration shellsand create local pinning zones reducing the probability ofwater molecules to slip even on hydrophobic surfaces.


Effect of Salt Concentration on Slip over HydrophobicSurfacesAnalogy

The game of Land and Water (Kumir Danga)

Difficult to swapislands with

larger number ofcrocodiles in

water


Comparison of Near-wall Water Density Profiles

Near wall densities variation of water next to a hydrophobic wall forinterparticle distance values of (a) D=5.5 (b) 4.5 (c) 3.5.

Near wall densities of water for different values of saltconcentration; (a) 0.2 M (b) 0.6 M (c) 1.0 M..

Decreasing D means increase inthe number of Islands as

visualized by the increasingvicinity between the patches

Nearer islands are alsoaccompanied by larger number

of water molecules at theinterface (more kids willing to

play the easy game) as visualizedby the change in the averagecolor of the number density

profile.

Ions at the interface tend toform pinning zones which

prevent both agglomeration ofwater molecules with high

number density as well as theirfree motion

Increasing ions with hydratedwater opposes the action ofshear in drifting the water

molecules (a strtech of waterhighly infested with crocodiles)


Combined Variation of D and Concentration

28

Based on the density variations of interfacial water molecules, we canmake the following assertions:

• Decreasing D reduces the distance between the near wall maximaof water molecules.• Decreasing D also increases the density of water molecules next tothe surface. This occurs due to enhanced screening of the waterrepelling sites on the wall by the now closer density maxima.• With increasing salt concentration the interfacial sites have moreand more immobile ions and pinned water molecules.

• Adsorption sites on a substrate canalter the interface fluid structurebetween dense and rarified phases• Addition of salt can similarly alterthe interface structure, making theinterface fluid density low byobstructing the adsorption sites


AnalogyThe game of Land and Water (Kumir Danga)

Extremely difficult toswap islands withlarger number of

crocodiles in waterand larger number ofkids irrespective of

the nearness ofislands

Combined Variation of D and Concentration


How to Achieve Stick on Hydrophobic Surfaces ?

30

• The enhanced near wall density with lower D and enhanced pinning ofinterfacial water at higher salt concentration can be combined to achievesignificant decrease in slip length of water molecules• For optimum combination of interparticle distance and salt concentration multi-layer sticking of fluid molecules can be achieved.

• Slip lengths vary with compositevariation of solute concentration andinterparticle distance.• The monotonic decrease in slip lengthwith increase of concentration and theabrupt drop in slip length at anintermediate solute molarity can beoptimized to obtain fluid stickirrespective of the contact angle.• The drop is steeper for lower valuesof the interparticle distance at the solidwall.• For a particular combination of thesalt concentration and the interparticledistance, water sticks on a hydrophobicsurface.


Some Theoretical Perspective

Fluid sticking or slipping at a solid interface is attributed to the energy dissipationoccurring due to the fluid solid interaction.MKT describes fluid molecules jumping between the adsorption s

2

ites separatedby distance2 with a frequency to facilitate interfacial transport.

Molecular diffusion coefficient ,is of the order , where the coefficient of

molecular friction ,is given by

o

mol

a

D U

3 , and being the Boltzmann

constant and absolute temperature respectively, and U being the velocity scaleAltering the lattice spacing would essentially alter the distribution of the surfa

BB

o

k Tk T

a

ceadsorption / desorption sites. This change would be reflected in the molecular friction,

culminating in altered molecular diffusion, and finally the macroscopically observed slip.


Theoretical Perspective (Contd.)

At the molecular level, this transition from a denser zone to a relatively rarifiedzone can be approximately visualized by means of the existence of an interfacebetween two fluids. This interface may becharacterized by an interfacial tension .The spatial gradient in this surface tension would lead to some additional

tangential stress which would either augment or diminish the intrinsic wall

s

shearstress and would according lead to slip or stick.The tangential stress balance equation at the interface of the dense and rare zones

of fluidin equilibrium can be written as n trd ( ) 0:where

and are the stress tensors for dense and rare phases respectively. and are the

unit vectors in directions normaland tangential to the wall respectively, and

grad ts s

n tr d

grads ( . )is the component of gradient operator in the local plane of interfacen n


Theoretical Perspective (Contd.)

' 'With salt, reduction in the interfacial tension is : whereis the modified surface tension, is the interfacial concentration of solute,R is the universal gas constant, and T is the abs

RTs s s

olute temperature.This added surface tension force tends to influence the drag at

the solid fluid interface, so that ( ) ,intwhere is the change of slip velocity at the interfa

u grad t As s sHus ce and is theint

change in theinterfacial area between the high and low density phases.1 , so that there is an inverse proportionality betweenint

the interparticle distance and slip length

A

Aa


Summary• We demonstrate that the relationship between intrinsicwettability and stick/slip is not obvious.• Contact angle is a parameter that ideally defines the fluidunder static conditions as derived from the chemicalproperty and physical structure of the solid surface.• Under dynamic conditions, however, apart from theproperties of the solid surface, the interfacial structuration inthe fluid phase also determines the effective wettability.• Thus, a hydrophobic surface is not synonymous with slipneither a hydrophilic surface means sticking of water.• Wettability can be a dynamic parameter that can bealtered without altering the chemical composition of thesurface.


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